As, for obscure reasons, I am replacing Roy Hattersley at the Kempsford Literary Festival and talking about Build Your Own Time Machine (if you're interested it's in St Mary's Church, Kempsford, 4.30pm on Sunday 27 April, £5 - tickets from kemplitfest@gmail.com or on 01285 810588, or on the door), I thought I'd do a quick time travel related post here. As I describe in the book/talk, there is nothing in the laws of physics that prevents time travel, and relativity makes forwards time travel relativity simple. Backwards is a lot more tricky, but in principle this is possible using general relativity effects. But many physicists believe that it can never actually happen, in part because of the paradoxes that arise if you can travel into the past. The best known time paradox is the so-called grandfather paradox, where you visit the past, kill a grandparent before you are born (I don't know why it's a grandparent, but it's traditional) and you're in ...